Nah they wouldn't use WMD as an excuse, the US is still ashamed of killing over one million Iraqis while searching for them, and in the end, they didn't find any.
We did find chemical weapons and the remains of nuclear research programs. We also know that Iraq was working with Russian elements for the development of bio weapons (of the viral type) with the use of mobile laboratories.
Of course - we didn't find stockpiles of chemical weapons, because that's not how chemical weapons work. You mix the components together, they are volatile for a few hours (some of them a little longer depending upon manufacturing quality and storage conditions), and then they break down. So you don't mix them until you use them.
The stigma is that we didn't find any because there weren't any SCUD missiles blowing up and gassing our soldiers.
As far as the evidence for nuclear programs - there are certain parts of various processes that require machines and implements of very specific and unique design tolerances that have been found. Again - no logical person expected to walk in and find rooms full of uranium centrifuges or unregistered breeder reactors.
The boots on the ground who looked at the suspect facilities and the guys watching the feeds from our satellites knew exactly what was going on there. The media created unreasonable standards of proof that were pure fantasy, then claimed there was nothing to see, no weapons, etc.
Which that actually was just as well. Since we know they went to Syria. You don't drive truck convoys out of countries like that without us knowing about it.
The thing is, No one will ever use the existence of WMD as an excuse for invading a country lol.
Countries don't need excuses for invading each other.
Realistically speaking - Saddam wasn't a direct threat to the U.S. He was not really even in control of his country when we attacked. The real concern was over the sale of chemical weapons and the development of bio weapons that were getting increased interest from groups like Aum Shinrikyo (who also had been trying to get people into Africa during outbreaks of Ebola).
Saddam's generals had largely gone rogue and were developing all kinds of goodies in exchange for some form of compensation (the oil helped throw chaff in the air over the relation). It was a sort of 'rogue' sub-state. Kind of interesting, really.
When a country such as the U.S. or Russia decides it's going to do something - it really doesn't much matter if it's considered justified, or not. It's happening.
The difference between U.S. and Russian culture is that the U.S. citizens expect explanations for everything their government does. Russia just does it with a loud cry of: "For Russia!"
Though it was kind of funny how Russia blatantly invaded while denying it. "Self defense forces... Yes... The Crimean people had this many young men with the same style of rifle, same military fatigues, and were regularly drilling with each other. .... No Russians troops, here. All proud Crimean patriots who speak Russian. But they aren't Russians. Or they would be wearing our flag. See... no flag. Well... They are Russians - they belong to Russia - that is why we must protect them in our country - but they are not Russians under the control of the Kremlin. Nope. That is just a conspiracy theory."
Why even put up the effort when everyone pretty much rolled their eyes and said: "Bullshit?"